1. Field of the Invention
The present invention relates to a process for producing a chloroprene elastomer capable of providing a vulcanized chloroprene rubber product having excellent heat aging resistance.
2. Description of the Prior Art
In some uses and under some use conditions, vulcanized rubber products are exposed to the atmosphere at a considerably elevated temperature and usually deteriorate with the lapse of time undergoing a change in properties. The changes depend upon the kind of elastomer used. There are two types of elastomers: one type being the so-called thermosetting type elastomers, the vulcanized products of which harden as the deterioration progresses to become ebonite ultimately; and the other type being the so-called heat-softening type elastomers, the vulcanized products of which soften as the deterioration progresses with deformation occurring. The function as a rubber elastic body is lost with both types of elastomers as a result of such deterioration.
It is desirable, needless to say, for rubber products to maintain an appropriate rubber elasticity at higher temperatures for a long period of time. Such a property is called heat aging resistance, which is one of the fundamental aging-resistance properties required for rubber including weather resistance, oil resistance and ozone resistance. Chloroprene rubber is called a thermo-setting deterioration-type rubber and usually exhibits a tendency toward a reduction in breaking elongation and an increase in hardness with the passage of its period of use with respect to rubber physical properties. The degrees thereof are unavoidably greater than that of main chain-saturated type rubbers such as acryl rubber, epichlorohydrin rubber, polyether rubber, etc., judging from the polymer structure thereof. However, it has been desired to further enhance the heat aging resistance of chloroprene rubber whose processability, vulcanization property, physical property, weather resistance and the like are well balanced.
Evaluation of physical properties of chloroprene rubber can be conducted through the compounding according to JIS K6388, i.e., a so-called gum stock compounding. However, in the production of rubber products, compounding is usually effected in the presence of suitable amounts of compounding chemicals such as carbon black, plasticizer, anti-oxidant, filler, and the like. Thus, the heat aging properties of rubber products are different from each other since the starting rubber, the compounding or the environment of use is different. Therefore, in rating the intrinsic properties of the starting rubber, it is simple and convenient to prepare a vulcanized sheet through an appropriate test compounding and to measure the changes in property under common deterioration conditions.
For the above-described reasons, in the present invention the heat aging resistance is rated according to the following method. That is, a sample of a No. 3 dumb-bell described in JIS K 6301, which is vulcanized from a given test compound, is left in a Geer aging tester for aging. The heat aging resistance is compared in terms of the degree of change in the retention ratio of the breaking elongation to the original value thereof with the lapse of time. With thermosetting deterioration type elastomers such as chloroprene rubber, those which have a greater retention ratio, i.e., a smaller change thereof, are better.
The heat resistance of chloroprene rubber is classified as showing a retention ratio of not less than 50% under the aging conditions of 100.degree. C, 70 hours, e.g., in JIS K 6403. This is not the highest resistant temperature of chloroprene rubber but, in the rubber industry, a temperature of 120.degree. C is commonly taken as a limit. Improvement of the heat resistance through compounding of rubber is described in, e.g., Kanenari Goda, Chloroprene Rubber, p. 45, Taiseisha, (1972). It is considered that improvement in the property of the polymer itself will lead to further improvement in the heat aging resistance with the help of improvement in the compounding. However, in conventional investigations on the production of chloroprene rubber, there are known no reports which suggest improvement of the heat resistance of the polymer itself under severe aging conditions.